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The effects of creep and fatigue on damage accumulation in ligament Schwab, Timothy David


Ligaments are collagenous tissues that passively resist tensile forces to maintain joint stability and guide joint motion. When ligaments are damaged or are healing from an injury, they have increased laxity. Increased ligament laxity may cause increased joint laxity, which could lead to other musculoskeletal complications. As damage is thought to influence ligament laxity, it is important to understand damage mechanisms and accumulation in ligament. Ligaments can be damaged during loading and in vivo loads can be either constant (creep) or cyclic (fatigue); therefore, the effects of creep and fatigue on damage accumulation were investigated in ligament. Medial collateral ligaments were subject to in vitro, uniaxial creep or fatigue tensile loading. Testing was completed at three maximum stresses: 60%, 30% and 15% of the ultimate tensile strength. Some ligaments were loaded until rupture, and some did not rupture during loading. Behaviour of ligaments subjected to creep was compared to those subjected to fatigue at each stress level. Ligament behaviour was quantified with times-torupture, strain profiles, and stress-strain characteristics. Damage was confirmed with a stretch to failure for those ligaments that did not rupture in creep or fatigue. Fatigue loading was more damaging than creep loading. Fatigue ligaments failed sooner than creep ligaments and the strain profiles differed between the two loading profiles. On a normalized time scale, creep ligaments experienced higher strains than fatigue ligaments. When time of loading was accounted for, fatigue ligaments failed sooner, which resulted in greater strains in fatigue than creep at specific time-points. Both creep and fatigue loading resulted in changes in stress-strain characteristics. Tangent modulus was used to track damage and inputted into a continuum damage mechanics model to predict time-to-rupture of creep and fatigue. Results indicate that fatigue loading likely involves an additional damage mechanism not present in creep loading. In essence, fatigue loading involves both cycle-dependent and time-dependent damage, while creep loading only involves time-dependent damage. Therefore, loading condition (static versus cyclic) was found to affect damage accumulation in ligament. Information from this study furthers the current understanding of ligament damage accumulation.

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